The present invention relates to a seal active clearance control system for a gas turbine stationary blade.
In a stationary blade of a gas turbine, the air of a compressor is partially bled from an outer shroud and guided through the inside of the stationary blade into a cavity of an inner shroud to make the pressure in the cavity higher than that of an outside hot combustion gas to thereby prevent the entrance of the hot gas into the inside.
FIG. 3 is a section showing a general sealing structure for the gas turbine stationary blade. In FIG. 3, a stationary blade 21 includes an outer shroud 22 and an inner shroud 23. This inner shroud 23 supports a seal ring retaining ring 24 at its flange, and a seal ring 25 is supported by the seal ring retaining ring 24 to seal discs 33a and 33b on the rotor side. A cavity 26 is formed by the seal ring retaining ring 24 and the inner shroud 23. Numeral 27 designates a hole formed in the seal ring retaining ring 24, and a sealing air tube 28 is formed through the stationary blade from the outer shroud 22 to the inner shroud 23.
Moving blades 31a and 31b are arranged adjacent to each other across the stationary blade 21 with respect to the longitudinal direction of the rotor axis, and have platforms 32a and 32b. Spaces 34 and 35 are formed in the stationary blade 21 between the moving blades 31a and 31b. Seal portions 36 and 37 at the two ends of the inner shroud 23 individually seal the platforms 32a and 32b of the moving blades and the two end portions of the inner shroud 23 of the stationary blade 21.
In the stationary blade thus constructed, a portion of bleed air of a compressor (that is, the sealing air 40) is guided from the compartment to the outer shroud 22 and flows through the sealing tube 28 in the stationary blade 21 and further into the cavity 26, as indicated by arrow 40a. A portion of the air having flown into the cavity 26 flows through the hole 27 of the seal ring retaining ring 24 into the front space 34, as indicated by arrow 40b, and further through the seal portion 36 into a combustion gas passage, as indicated by arrow 40c. Moreover, the sealing air passes the seal portion of the seal ring 25 and flows into the rear space 35, as indicated by arrow 40d, until it finally flows out from the rear seal portion 37 to the combustion gas passage, as indicated by arrow 40e.
Due to the sealing air 40 described above, the pressure in the cavity 26 formed in the inner shroud 23 and in the two spaces 34 and 35 is made higher than that in the combustion gas passage to prevent the hot combustion gas from entering the inside of the inner shroud 23.
On the other hand, a clearance .delta.H has to be maintained between the confronting faces of the seal ring 25 of the stationary portion and the rotor discs 33a and 33b of the rotary portion. An excessively large clearance .delta.H increases the leakage of air and lowers the sealing performance, and an excessively small clearance .delta.H causes the stationary side and the rotary side to contact each other. Thus, it is necessary to set a proper clearance.
On the inner side of the stationary blade of the gas turbine, as described above, there is mounted the seal ring 25 to keep the clearance .delta.H at the face confronting the rotor disc portion of the rotary portion. This clearance .delta.H may increase the leakage, if excessively large, so as to affect the sealing performance adversely, and may also cause, if excessively small, the stationary portion and the rotary portion to contact each other.
This clearance .delta.H is extended or contracted due to the influences of thermal elongation of the rotary portion and the stationary portion in the running state of the gas turbine during, for example, a starting time or a loaded running time. This thermal elongation is slightly different between the stationary portion and the rotary portion, but the clearance .delta.H has to be set so that no contact may occur between the two portions at the minimum clearance during the run. Usually, the clearance .delta.H is set with an allowance to keep the portions from contacting even when it is minimized at an assembly time. However, this clearance has to be set as small as possible, while sufficient for avoiding the contact. At present, however, there is no means for controlling the clearance properly, and it has been earnestly desired to realize such means.